Cucumber mosaic virus (CMV) is a single-stranded (+) RNA plant virus which has a functionally divided genome. The virus genome contains four RNA species designated RNAsl-4;3389 nucleotides (nt), 3035 nt, 2193 nt and 1027 nt, respectively (Peden and Symons, 1973; Gould and Syrnons, 1982; Rezaian et al., 1984; Rezaian et al., 1985). Only RNAsl-3 are required for infectivity (Peden and Symons, 1973) because the coat protein, which is encoded by RNA 4, is also encoded by RNA 3. Translations of CMV RNAs yield a 95KDal polypeptide from RNA 1, a 94kDal polypeptide from RNA 2, (Gordon et al., 1983) and two polypeptides from RNA 3: its 5' end encodes a 35KDal polypeptide, and its 3' end encodes a 24.5kDal polypeptide (Gould and Symons, 1982). The 24.5kDal polypeptide is identical to dial encoded by RNA 4 and is the coat protein.
Several strains of cucumber mosaic virus have been classified using serology (Devergne and Cardin, 1973, 1975), host range (Marrow et al. 1975), peptide mapping (Edwards and Consalves, 1983), and nucleic acid hybridization (Piazzola et at., 1979; Gonda and Symons, 1978). These CMV strains can be divided into two groups designated S and WT. The genome of the CMV-Q strain has been completely sequenced (Rezaian et al., 1984, 1985; Gould and Symons. 1982; Davies and Symons, 1988). The Q strain is a member of the S group, which consists of three members. The WT group is Iraown to contain at least 17 members. From nucleotide sequence analysis and comparisons of the coat protein genes from CMV-C and CMV-WL (Quemada et al, manuscript in preparation; see Chart 1) we have determined that the C strain belongs to the WT group while the WL belongs to the S group. The nucleotide and amino acid sequences of the coat protein genes from strains C and WL differ by 22.7% and 16%, respectively (see charts 2 and 3).
As has been shown for several viruses [tobacco mosaic virus (Powell-Abel et at., 1986), alfalfa mosaic virus (Loesch-Fries et al., 1987; Tumer et al., 1987), cucumber mosaic virus (Cuozzo et al., 1988; Quemada and Slightore, in preparation), and potato virus X (Hemenway et al., 1988)]expression of the coat protein in transgenie plants results in a plant which is resistant to infection by the respective virus. However, whether this engineered cross-protection will extend to all strains of a particular virus has not been determined. The two CMV groups appear to differ in their coat protein gene by about 16%, thus it is possible that the expression of both virus coat proteins may be needed to ensure engineered cross-protection against CMV infections which could be expected under field conditions.
The CMV coat protein gene does not contain the signals necessary for its expression once transferred and integrated into a plant genome. It must be engineered to contain a constitutive promoter 5' to its translation initiation codon (ATG) and a poly(A) addition signal (AATAAA) 3' to its translation termination codon. Several promoters which function in plants are available, but we believe that the best promoters is the 35S constitutive promoters from cauliflower mosaic virus (CaMV). The poly (A) signal can be obtained from the CaMV 35S gene or from any number of well characterized plant genes, i.e. hopaline synthase (Bevan et al.,1983), octopine synthase (Depicker et at., 1982), and the bean storage protein gene phaseolin (Slightore, et at., 1983). The constructions are similar to that used for the expression of the CMV-C coat protein in PCT Patent Application PCT/U.S. Pat. No. 88/04321, published on Jun. 29, 1989 as WO 89/05858, claiming the benefit of U.S. Ser. No. 135,591, filed Dec. 21, 1987, now abandoned, entitled "Cucumber Mosaic Virus Coat Protein Gene", the essential pans which are herein incorporated by reference.